Enter the text here that is the new abstract information for your application. This section must be no longer than 30 lines of text. Crossovers direct the accurate segregation of meiotic chromosomes. Several decades of effort has led to a detailed model outlining the mechanism of meiotic recombination. This is based almost entirely on research done with a single model organism, the budding yeast S. cerevisiae. We recently tested key features of this model in the metazoan Drosophila melanogaster. To do this, we knocked out both canonical (long- patch) and short-patch mismatch repair, the first time this has been done in a metazoan. Our results suggested a new model, one that has some fundamental differences from the budding yeast model. In Aim 1 we test key predictions of our model. To do this, we examine genome-wide meiotic recombination in the absence of mismatch repair. We will sequence maternally haploid embryos and also maternally haploid embryos that are disomic for one major chromosome, which is essentially half-tetrad analysis. We've also made recent discoveries that open up a new avenue to understanding how crossover position is controlled. We found that the Bloom syndrome helicase (BLM) and a complex we call the meiotic-MCM (mei-MCM) complex are both required for normal crossover positioning; in mutants, crossovers are distributed randomly along a chromosome arm and even occur on chromosome 4, which normally has no meiotic crossovers. We hypothesize that BLM controls choice of repair pathway; we test this hypothesis in Aim 3. We found that the mei-MCM complex is physically required for normal crossover regulation, but this does not require ATPase activity (this activity is essential for making crossovers, just not for distributing them). In Aim 3 we continue to elucidate functions of the mei-MCM complex in crossover control, including blocking crossover near the centromere and on chromosome 4. Results from these experiments will shed new light on meiotic recombination mechanisms and provide novel insights into regulation of crossover placement.